How many times can we create a master field on multiple child objects?
means 
Account is the parent object and if i have 200  objects or more or less, then i want to create a field called AccountId on all 200 objects using master-detail relationship.how can i acheive this?

hi,
i created one report and gave that report access to community users those have customer community plus  license only but they are able to see other account's records also means i have two accounts,one account has 5 users and another account has another 3 users.out of the 5 users of 1st account,two users have report access and remaining are using customer community license only.but here is a problem,i can not restrict the records which don't belong to that account,the two users can see other account records also.they are using one common object(Order__c) only how can i solve this....? 
anyone help me please.....

How many times can we create a master field on multiple child objects?
means 
Account is the parent object and if i have 200  objects or more or less, then i want to create a field called AccountId on all 200 objects using master-detail relationship.how can i acheive this?

hi,
i created one report and gave that report access to community users those have customer community plus  license only but they are able to see other account's records also means i have two accounts,one account has 5 users and another account has another 3 users.out of the 5 users of 1st account,two users have report access and remaining are using customer community license only.but here is a problem,i can not restrict the records which don't belong to that account,the two users can see other account records also.they are using one common object(Order__c) only how can i solve this....? 
anyone help me please.....